1. 11.2 Energy and Power

2. Chapter 11 Objectives  Give an example of a process and the efficiency of a process.  Calculate the efficiency of a mechanical system from energy and work.  Give examples applying the concept of efficiency to technological, natural and biological systems.  Calculate power in technological, natural, and biological systems.  Evaluate power requirements from considerations of force, mass, speed, and energy.  Sketch an energy flow diagram of a technological, natural, or biological system.

3. Chapter 11 Vocabulary  carnivore  cycle  decomposer  ecosystem  efficiency  energy conversions  energy flow  food calorie  food chain  food web  herbivore  horsepower  irreversible  power  power transmission  producer  reversible  steady state  watt

4. Inv. 11.2 Energy and Power Investigation Key Question: How powerful are you?

5. 11.2. Energy and Power  How fast you do work makes a difference.

6. 11.2 Power  Power is equal to the amount of work done divided by the time it takes to do the work. P = E t Change in work or energy (J) Change in time (sec) Power (W)

7.  You are asked for power.  You are given mass, distance, and time.  Use E p = mgh, P= E ÷ t  Solve E p = (70 kg) (9.8 N/kg) (5 m) = 3,430 J  Solve P = (3,430 J) ÷ (30 s) = 114 watts  114 watts  This is a little more than a100 watt light bulb. Calculating power A 70 kg person goes up stairs 5 m high in 30 sec. a) How much power does the person need to use? b) Compare the power used with a 100-watt light bulb.

8. 11.2 Power  A unit of power is called a watt.  Another unit more familiar to you is horsepower.  One horsepower (the avg. power output of a horse) is equal to 746 watts.

9. 11.2 Power  Another way to express power is as a multiple of force and it's velocity, if the velocity and force are both vectors in the same direction. Velocity (m/sec) Force (N) Power (W) P = F. v

10. 11.2 Power in human technology  You probably use technology with a wide range of power every day.  Machines are designed to use the appropriate amount of power to create enough force to do work they are designed to do.

11.  You are asked for power.  You are given volume, density, speed and time.  Use density = m ÷ V, E k = ½ mv 2, P = E ÷ t  Solve: m = (1 kg/m 3 ) (2 m 3 )= 2 kg  Solve E k = (0.5) (2 kg)(3m/s) 2 = 9 J  With 10% efficiency, it takes 90 J input energy to make 9 J output, solve: P = 90 J ÷ 1 s = 90 W Estimating power A fan uses a rotating blade to move air. How much power is used by a fan that moves 2 m 3 of air each second at a speed of 3 m/sec? Assume air is initially at rest and has a density of 1 kg/m 3. Fans are inefficient; assume an efficiency of 10 %.

12. 11.2 Power in natural systems  Natural systems exhibit a much greater range of power than human technology  The sun has a total power output of 3.8 × 10 26 W.  The power received from the sun is what drives the weather on Earth.

13. 11.2 Power in biological systems  200 years ago, a person’s own muscles and those of their horses were all anyone had for power.  Today, the average lawn mower has a power of 2,500 watts—the equivalent power of three horses plus three people.  Most of the power output of animals takes the form of heat.  The output power from plants is input power for animals.

14.  You are asked for power.  You are given energy input in food calories and time.  1 day = 86,400 s, 1 food calorie = 4,187 J, use P = E ÷ t  Solve: E = (2,500 cal) (4,187 J/cal) = 10,467,500 J  P = (10,467,500 J) ÷ (86,400 s) = 121 watts Estimate power An average diet includes 2,500 food calories/day. Calculate the average power this represents in watts over a 24-hour period. One food calorie equals 4,187 joules.